Phase shifts to algal dominated communities are an important threat to coral reefs around the globe. As more algae dominate space on reefs they have potential to disrupt natural processes that drive reef resistance to stress. However, the mechanisms that drive these patterns are rarely studied. Recent collaborations have shown that younger coral colonies are more susceptible to algal competition and sublethal stress than adult colonies (Olsen et al., 2014). This competition can be driven by crude extracts from brown algae that cause allelopathy (Paul et al., 2011). We have also found that the larval stages of some coral are susceptible to competition from algae and cyanobacteria (Kuffner et al., 2006). Together this shows that competition with macroalgae has great potential to reduce coral recruitment, a critical process for reef recovery.

In addition to phase shifts, marine communities will be exposed to higher seawater temperatures as the climate continues to change. A major question in marine ecology is how multiple stressors affect ecological interactions and community dynamics. My research has tested how elevated seawater temperatures might interact with algal competition to drive community shifts. Our studies have found that small temperature changes can cause sublethal stress to corals and their larvae (Olsen et al., 2013, 2014). This sublethal stress can cause latent effects impacting post-settlement survival (Ross et al., 2013). Preliminary experiments showed that a sublethal temperature stress did not interact with brown algae to change interaction dynamics (Olsen et al., 2014), but further work with an individual compound isolated from cyanobacteria showed that there is a significant interaction between temperature and allelopathy to reduce coral larval survival (Ritson-Williams et al., submitted). The interaction of multiple stressors is an active area of my research and will be important for us to understand coral reef resistance to local and global stressors.

Critical to understanding the ecology of organisms is understanding how they survive. Many marine organisms use chemical defenses to protect themselves. I have used multiple marine organisms to test the ecology of chemical defenses. In collaboration with Dr. Valerie Paul I’ve tested a large suite of algae to determine which ones were protected by secondary metabolites. These experiments involve extraction of secondary metabolites and adding them to artificial food and feeding this to a natural assemblage of reef fish and to sea urchins, both potential consumers of algae (manuscript in prep). We showed that many of these algae use secondary metabolites for protection from consumers, which is an important mechanism for algae to persist on degraded reefs. As part of these experiments we also tested the chemical defenses of marine benthic cyanobacteria (Mathew et al., 2010). These benthic organisms are rich producers of novel natural products (Gunasekera et al., 2008) and are rarely eaten by marine consumers.

In addition to algae and cyanobacteria I’ve tested the chemical defenses of sponges and nudibranchs. Secondary metabolites isolated from the sponge Acanthella cavernosa were potent chemical defenses against a natural assemblage of reef fish. The predatory nudibranch Phyllidiella elegans eats this sponge and sequesters the compounds to use as its own chemical defense. These nudibranchs are brightly colored and through extensive field experiments I showed that both color patterns and chemical defenses can protect marine organisms from predation, showing the evolutionary advantage of aposematic coloration (Ritson-Williams and Paul, 2007).

Secondary metabolites can also be critical as toxins for prey capture. I found an undescribed species of flatworm (Planocera sp. 1) that consumes mobile prey. This worm eats many different types of prey and was shown to use the compound tetrodotoxin and a novel analog to kill its prey (Ritson-Williams et al., 2006; Yotsu-Yamashita et al., 2013). Tetrodotoxin is often assumed to be a chemical defense and has been shown to be a pheromone, but my study is the first to document its use as a prey capture toxin. This is an exciting discovery as many different species have tetrodotoxin but the ecological functions of this compound are rarely tested. This type of evolutionary ecology research is very exciting as it shows the key role some secondary metabolites have in organisms’ survival and persistence.

An important aspect of marine chemical ecology is chemical settlement cues. Many marine invertebrates have a larval dispersal phase and how they find appropriate settlement substrata is critical for their survival and persistence. This is especially apparent in specialized predators. Nudibranchs in the genus Phestilla are specialist predators on different species of corals. Through a series of ecological experiments I* showed that 4 species of nudibranchs of Phestilla partition their niches by specializing on distinct coral species (Ritson-Williams et al., 2003). While these species have different life history characteristics (Ritson-Williams et al., 2007) they all require water-soluble cues from their prey to induce settlement and metamorphosis (Ritson-Williams et al., 2009). This type of obligate settlement cue makes sense for a specialist predator that has a narrow diet breadth. This project shows the important of chemical cues for specialists to find their prey species, again illustrating the influence of chemical cues on organismal evolutionary ecology.